U.S. patent application number 09/998237 was filed with the patent office on 2002-08-22 for photosensitive flexographic printing element having at least two ir-ablative layers.
Invention is credited to Kaczun, Jurgen, Philipp, Sabine.
Application Number | 20020115019 09/998237 |
Document ID | / |
Family ID | 7666312 |
Filed Date | 2002-08-22 |
United States Patent
Application |
20020115019 |
Kind Code |
A1 |
Kaczun, Jurgen ; et
al. |
August 22, 2002 |
Photosensitive flexographic printing element having at least two
IR-ablative layers
Abstract
A photosensitive flexographic printing element for the
production of flexographic printing plates by digital imaging by
means of lasers which comprises a combination of at least two
different IR-ablative layers in which one layer comprises an
elastomeric binder and the other comprises a self-decomposing
binder. A process for the production of flexographic printing
plates using an element of this type.
Inventors: |
Kaczun, Jurgen;
(Niederkirchen, DE) ; Philipp, Sabine;
(Morfelden-Walldorf, DE) |
Correspondence
Address: |
Herbert B. Keil
KEIL & WEINKAUF
1101 Connecticut Ave., N.W.
Washington
DC
20036
US
|
Family ID: |
7666312 |
Appl. No.: |
09/998237 |
Filed: |
December 3, 2001 |
Current U.S.
Class: |
430/273.1 ;
430/271.1; 430/306; 430/944; 430/945 |
Current CPC
Class: |
G03F 7/202 20130101;
B41C 1/05 20130101 |
Class at
Publication: |
430/273.1 ;
430/945; 430/306; 430/271.1; 430/944 |
International
Class: |
G03F 007/095; G03F
007/18 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 7, 2000 |
DE |
10061116.8 |
Claims
We claim:
1. A photosensitive flexographic printing element for the
production of flexographic printing plates for digital imaging by
means of lasers, comprising at least a dimensionally stable
support, at least one photopolymerizable layer, at least comprising
an elastomeric binder, a polymerizable compound and a
photoinitiator or photoinitiator system, at least two
laser-ablatable layers A and B, each comprising at least one binder
and also an IR absorber for laser radiation, and optionally a
removable, flexible protective film wherein the at least one binder
of layer A is an elastomeric binder and the at least one binder of
layer B is a self-decomposing binder, and the optical density of
the entire layer sequence of IR-ablative layers in the actinic
spectral region is at least 2.5.
2. A photosensitive flexographic printing element as claimed in
claim 1, wherein the self-decomposing binder of layer B contains
nitro or nitrate ester groups.
3. A photosensitive flexographic printing element as claimed in
claim 2, wherein the binder containing the nitro and/or nitrate
ester groups is a cellulose or cellulose ether nitrate ester.
4. A photosensitive flexographic printing element as claimed in one
of claims 1 to 3, wherein the elastomeric binder is a binder
comprising diene units.
5. A photosensitive flexographic printing element as claimed in one
of claims 1 to 4, wherein the IR absorber is carbon black.
6. A photosensitive flexographic printing element as claimed in one
of claims 1 to 5, wherein the flexographic printing element has
further IR-ablative layers.
7. A process for the production of a flexographic printing plate in
which the starting material employed is a photosensitive
flexographic printing element as claimed in one of claims 1-6,
comprising the following steps: (a) removal of the removable,
flexible protective film, if present, (b) writing of a mask into
the layer system comprising IR-ablative layers by means of an IR
laser, (c) full area exposure of the photosensitive element to
actinic light through the mask formed in step (b), (d) treatment of
the intermediate formed in (c) with at least one developer
solution, during which the residues of the IR-ablative layers which
have not been removed in step (b) are removed and the exposed
photopolymerizable layer is developed.
8. A process as claimed in claim 7, wherein step (b) is carried out
using a laser apparatus having a rotating drum, and the
flexographic printing element is mounted on this drum for ablation.
Description
[0001] The present invention relates to a photosensitive
flexographic printing element for the production of flexographic
printing plates by digital imaging by means of lasers which
comprises a combination of at least two different IR-ablative
layers in which one layer comprises an elastomeric binder and the
other comprises a self-decomposing binder. The invention
furthermore relates to a process for the production of flexographic
printing plates using an element of this type.
[0002] The conventional method for the production of flexographic
printing plates by laying a photographic mask on a photopolymeric
recording element, irradiating the recording element with actinic
light through this mask and washing out the unpolymerized areas of
the exposed element with a developer liquid is increasingly being
replaced by CtP (computer-to-plate) technology, frequently also
known as digital imaging. In CtP methods, the photographic mask in
conventional systems is replaced by the masks integrated into the
recording element.
[0003] Although a number of different methods have been proposed,
only two have hitherto achieved significant importance in the
market. In the first method, the photopolymerizable element is
provided with a suitable ink receptor layer, and a mask is printed
on by means of an ink-jet printer, as disclosed, for example, in WO
97/25206. The element can subsequently be exposed and developed in
a known manner.
[0004] In the second method, the photopolymerizable element is
coated with a substantially opaque, IR-ablative layer. Layers of
this type usually comprise carbon black. Imagewise irradiation by
means of a laser removes the IR-ablative layer at the points where
it is hit by the laser beam, and the underlying photopolymerizable
layer is uncovered. The image recording element can then be
irradiated over its full area with actinic light through the
ablatively formed mask in a known manner and washed out using a
developer liquid. In the washing-out step, the nonablated residues
of the IR-ablative layer and the underlying unpolymerized areas of
the exposed element are removed.
[0005] Flexographic printing elements having IR-ablative layers are
known in principle. EP-A 654 150 discloses a flexographic printing
element having an IR-ablative layer. The IR-ablative layer
comprises an IR-absorbent material. In addition, polymeric binders
and a large number of different auxiliaries, for example dispersion
aids or plasticizers, are disclosed as optional constituents.
Furthermore, an additional barrier layer between the
photopolymerizable layer and the IR-ablative layer is disclosed.
This is intended to prevent diffusion of monomers from the
photopolymerizable layer into the IR-ablative layer. EP-A 654 150
also discloses the fact that, in principle, more than one
IR-ablative layer can be available.
[0006] EP-A 741 330 discloses an IR-ablative flexographic printing
element which has no barrier layer of this type. A multiplicity of
widely differing polymers is disclosed as binders for the
IR-ablative layer. Furthermore, the IR-ablative layer may also
comprise a second binder in a smaller amount, for which a
multiplicity of widely varying polymers is likewise disclosed.
[0007] EP-A 908 778 discloses a flexographic printing element which
has an IR-ablative layer comprising SIS or SBS block
copolymers.
[0008] EP-A 767 407 discloses a flexographic printing element
having an IR-ablative layer which has an elastomeric, film-forming
binder. Binders disclosed are polyamides and polyvinyl alcohol
polyethylene glycol graft copolymers.
[0009] In the process for the production of flexographic printing
plates by IR ablation, the quality of the IR-ablative layer is the
crucial parameter for the quality of the flexographic printing
plate and the economic efficiency of the process. The IR-ablative
layer must satisfy a number of widely varying quality criteria:
[0010] It should have high sensitivity to lasers in order to ensure
rapid and complete removal of the layer with the lowest possible
laser power.
[0011] It should be soluble in conventional wash-out agents for the
photopolymerizable layer so that it can be removed together with
the unpolymerized constituents of the layer during the conventional
development step. Otherwise, two wash-out steps would have to be
carried out.
[0012] The laser apparatuses used nowadays are usually instruments
with rotating drums (external or internal drums). The IR-ablative
layer must therefore be elastic in order that it does not tear or
wrinkle on clamping onto the drums.
[0013] It must be tack-free in order that no dust is attracted
which could interfere with the IR ablation.
[0014] For storage and transport, flexographic printing elements
are usually protected against damage by means of a protective film,
which has to be removed before the IR ablation. The protective film
must have only low adhesion to the IR-ablative layer in order that
the IR-ablative layer is not damaged on removal.
[0015] Conversely, the IR-ablative layer must adhere strongly to
the light-sensitive layer in order that it is not removed at the
same time on removal of the protective film and in order that no
air bubbles nullify the advantage of direct contact between the
IR-ablative layer and the photopolymerizable layer.
[0016] The person skilled in the art who would like to produce a
high-sensitivity, high-quality flexographic printing element having
an IR-ablative layer sees himself confronted with a typical catch
22 situation. In order to obtain an IR-ablative layer with the
highest possible sensitivity, the use of a self-oxidative
self-decomposing binder, such as nitrocellulose, is advisable.
However, nitrocellulose layers are very brittle, and consequently
the elasticity of nitrocellulose layers is unsatisfactory and
flexographic printing elements of this type tear and wrinkle easily
on clamping onto drum instruments. Although the brittleness can be
reduced by addition of suitable plasticizers, the addition of
plasticizers frequently has the consequence, however, of tacky
layers with excessive cover film adhesion. However, typical
tack-free binders, such as certain polyamides, have significantly
lower sensitivity.
[0017] It is an object of the present invention to provide a
flexographic printing element whose IR-ablative layer has the
highest possible proportion of self-decomposing binders and yet
satisfies the abovementioned requirements.
[0018] We have found, surprisingly, that this objective can be
achieved by a combination of at least two different IR-ablative
layers.
[0019] Accordingly, a photosensitive flexographic printing element
for the production of flexographic printing plates for digital
imaging by means of lasers has been found which comprises at
least
[0020] a dimensionally stable support,
[0021] a photopolymerizable layer, at least comprising an
elastomeric binder, a polymerizable compound and a photoinitiator
or photoinitiator system,
[0022] at least two laser-ablatable layers A and B, each comprising
at least one binder and also an IR absorber for laser radiation,
and
[0023] optionally a removable, flexible protective film,
[0024] where the at least one binder of layer A is an elastomeric
binder and the at least one binder of layer B is a self-decomposing
binder, and the optical density of the entire layer sequence of
IR-ablative layers in the actinic spectral region is at least
2.5.
[0025] We have also found a process for the production of
flexographic printing plates using an element of this type.
[0026] In detail, the following comments should be made regarding
the invention.
[0027] In the photopolymerizable printing element according to the
invention, a conventional photopolymerizable layer, if desired with
an adhesion layer, is applied to a dimensionally stable support.
Examples of suitably dimensionally stable supports are plates,
films and conical and cylindrical tubes (sleeves) made from metals,
such as steel or aluminum, or plastics, such as polyethylene
terephthalate (PET) or polyethylene naphthalate (PEN).
[0028] The photopolymerizable layer consists of a negative-working
photopolymerizable mixture, i.e. one which is cured by exposure.
This can be carried out by photocrosslinking with previously
prepared polymers, by photopolymerization of low-molecular-weight,
photopolymerizable compounds or both. Photopolymerizable layers
essentially consist of a polymeric, elastomeric binder which can be
washed out in the developer, an ethylenically unsaturated,
free-radical-polymerizable compound, a photoinitiator or a
photoinitiator system, and optionally further additives and
auxiliaries. The composition of layers of this type is known in
principle and is described, for example, in DE-A 24 56 439 or EP-A
084 851.
[0029] The elastomeric binder can be a single binder or a mixture
of various binders. Examples of suitable binders are the known
vinylaromatic-diene copolymers or block copolymers, for example
conventional SIS or SBS block copolymers, diene-acrylonitrile
copolymers, ethylene-propylene-diene copolymers or
diene-acrylate-acrylic acid copolymers. Examples of suitable
polymerizable compounds are conventional ethylenically unsaturated
monomers, such as acrylates or methacrylates of mono- or
polyfunctional alcohols, acrylamides or methacrylamides, vinyl
ethers or vinyl esters. Examples include butyl (meth)acrylate,
2-ethylhexyl (meth)acrylate, butanediol di(meth)acrylate and
hexanediol di(meth)acrylate. Suitable initiators for the
photopolymerization are aromatic compounds, for example keto
compounds, such as benzoin or benzoin derivatives.
[0030] The photopolymerizable mixtures may furthermore comprise
conventional auxiliaries, for example inhibitors for thermally
initiated polymerization, plasticizers, dyes, pigments,
photochromic additives, antioxidants, antiozonants or extrusion
aids.
[0031] The precise composition and thickness of the
photopolymerizable layer is determined by the person skilled in the
art depending on the particular requirements. It is also possible
to employ a plurality of photopolymerizable layers of identical or
different composition arranged one on top of the other.
Furthermore, the element may comprise additional layers, for
example adhesion layers, upper layers or elastic underlayers.
[0032] The photopolymerizable layers may be developable in aqueous
or organic media, depending on the binder used. However, the
advantages of the invention are particularly evident if the layer
can be developed in organic media.
[0033] The element which is essential to the invention is the novel
combination of at least two different types of IR-ablative layers A
and B. These may be on the photopolymerizable layer, directly or
also indirectly, i.e. separated by a thin interlayer, for example
an adhesion or barrier layer. The IR-ablative layer is preferably
applied directly to the photopolymerizable layer.
[0034] The IR-ablative layer A comprises at least one elastomeric
binder and an IR absorber for laser radiation. Examples of
elastomeric binders which can be employed are binders which can
also be employed for the production of the photopolymerizable
layer. It is possible to use, in particular, polymers comprising
diene units, for example polybutadiene, polyisoprene or natural
rubber.
[0035] It is particularly advantageous to use block copolymers
having rigid polymer blocks comprising styrene, acrylonitrile or
acrylate units and elastic blocks comprising diene polymers, for
example butadiene or isoprene. Suitable are, for example,
elastomeric three-block copolymers having styrene-isoprene-styrene
or styrene-butadiene-styrene blocks, as described, for example, in
DE-A 22 15 090. The three-block copolymers can be employed as the
only elastomers or alternatively as a mixture with two-block
copolymers, for example styrene-isoprene or styrene-butadiene
two-block copolymers. Preference is given to SIS block copolymers.
The diene units can also be fully or partially hydrogenated.
[0036] It is also possible to employ other elastomers, for example
acrylate rubbers, elastomeric polyurethanes, for example
polyether-polyurethanes or polyester-polyurethanes, silicone
rubbers or elastomeric polyamides.
[0037] It is of course also possible to employ mixtures of two or
more different elastomeric binders.
[0038] The amount of binders A in the IR-ablative layer A is
generally from greater than 40% by weight to a maximum of 90% by
weight, based on the amount of all constituents of the IR-ablative
A layer, preferably from 45 to 80% by weight and very particularly
preferably from 45 to 70% by weight.
[0039] The IR-ablative layer B comprises at least one
self-decomposing binder and at least one IR-absorber for laser
radiation.
[0040] The self-decomposing or self-oxidative binder is a binder
which decomposes or depolymerizes very easily under the action of
heat without melting. Corresponding pixels therefore have very
steep edges, enabling very high resolutions to be achieved.
Self-decomposing or self-oxidative binders which can be employed
are binders which contain azide groups and very particularly
preferably those which contain nitro or nitrate ester groups.
[0041] Examples of suitable binders include polyglycidyl azide,
polyglycidyl nitrate or polyvinyl nitrate, polymers of
nitro-substituted styrenes, for example polymers containing nitro-,
dinitro- or trinitrostyrene groups. Polystyrene may furthermore
also be nitro-substituted in the main chains. Further examples
include polyacrylates and polymethacrylates, for example polymers
containing 2,4-dinitrophenyl acrylates or p-nitrophenyl acrylate as
monomeric units.
[0042] Particularly suitable self-decomposing binders are nitrate
esters of cellulose or cellulose derivatives, for example cellulose
ethers. Nitrate esters of this type are also known as
nitrocellulose and are commercially available with various contents
of nitrate ester groups (for example Wolff-Walsrode), the degree of
esterification affecting not only the decomposition properties, but
also the solubility properties of the nitrocellulose. The invention
is preferably carried out using alcohol-soluble nitrocellulose
grades. These are commercially available and usually have an N
content of from 10.9 to 11.3%. However, it is also possible to
employ other grades having a higher nitrogen content.
[0043] It is also possible to employ nitrate esters of cellulose
ethers, for example methylcellulose, ethylcellulose and in
particular 2-hydroxyethyl-, 2-hydroxypropyl- or
carboxymethylcellulose. Nitrated carboxymethylcellulose can also be
employed as Na salt, which increases the water solubility. It is of
course also possible to employ mixtures of various self-decomposing
binders.
[0044] The amount of self-decomposing binders in the IR-ablative
layer B is generally greater than 40 to a maximum of 90% by weight,
based on the amount of all constituents of the IR-ablative layer B,
preferably from 45 to 80% by weight and very particularly
preferably from 45 to 70% by weight.
[0045] Furthermore, both layers A and B comprise at least one
absorber for laser radiation.
[0046] The absorber is a substance which is finely distributed in
the layer. This should have the highest possible absorption between
750 and 20,000 nm. Suitable substances as absorber include
IR-absorbent dyes, for example phthalocyanines and phthalocyanine
derivatives, merocyanines or methine dyes, and strongly colored
inorganic pigments, for example carbon black, graphite, iron oxides
or chromium oxides. Preference is given to carbon black. Besides
the function as IR absorber, carbon black equally ensures that the
IR-ablative layer is opaque to actinic radiation, meaning that it
is not absolutely necessary additionally to employ a UV-absorbent
dye. It is preferred to employ finely divided particles in order to
obtain the highest possible tinting strength and the most uniform
layer possible. Preference is given to finely divided carbon black
grades, for example Printex.RTM. U, Printex.RTM. L6, Special Black
4 or Special Black 250 from Degussa.
[0047] The amount of absorber for the laser radiation is usually
from 1 to 59.9% by weight, based on the sum of all constituents of
the layer, preferably from 10 to 50% by weight and particularly
preferably from 25 to 50% by weight.
[0048] The IR-ablative layers A and B may additionally also
comprise additional auxiliaries or additives. Examples of
auxiliaries of this type are dispersion aids for pigments, fillers,
plasticizers or coating aids. Auxiliaries or additives of this type
are selected by the person skilled in the art depending on the
desired properties of the layer, provided that they do not
adversely affect the properties of the layer. Suitable dispersion
aids for carbon blacks are, in particular, polyoxyalkylene
derivatives, for example Solperse grades (Avecia) or block
copolymers, such as Disperbyk grades (Byk). It is also possible to
employ UV absorbers as auxiliaries. This is usually not absolutely
necessary in the case of the use of carbon black as absorber, but
may occasionally be advantageous. On use of IR dyes as absorber,
the use of additional UV absorbers is frequently unavoidable,
although not absolutely necessary in each case. In general, the
amount of all additives and auxiliaries should not exceed 20% by
weight, preferably 10% by weight and very particularly preferably
5% by weight, based on the sum of all constituents of the
IR-ablative layer.
[0049] In addition to the principal binder the IR-ablative layers A
and B may, moreover comprise at least one further binder of a
different chemical nature, which will be referred to below as the
secondary binder. It is also possible to employ a plurality of
different binders as secondary binders. The amount of the secondary
binder is less than the amount of the principal binder. The amount
of a secondary binder of this type is usually from 0 to 40% by
weight, preferably from 0 to 20% by weight and particularly
preferably from 0 to 10% by weight, based on all constituents of
the IR-ablative layer.
[0050] The secondary binder is selected by the person skilled in
the art depending on the desired properties of the IR-ablative
layer, provided that the properties of the layer are not adversely
affected by the secondary binder.
[0051] The sequence of layers A and B is not essential to the
invention. Layer A may be arranged as the lowermost layer, i.e.
directly on the photopolymerizable printing layer, and layer B on
top. However, the reverse layer sequence, namely B-A, can also be
selected particularly advantageously, with the layer comprising the
self-decomposing binder being embedded between the elastomeric
layer and the photopolymerizable layer. It is also possible for a
plurality of layers A and B to be present. Thus, for example, a
sandwich layer sequence A-B-A may be present, with the layer
comprising self-oxidative binder B being embedded between two
layers A.
[0052] Besides layers A and B, the flexographic printing element
may also comprise further, preferably IR-ablative layers of a
different composition, for example layers C. The layer sequence
A-B-C, B-A-C or C-B-A, for example, is then possible. Layers C of
this type may have a similar structure to A and B comprising binder
and IR absorber, but they may also have a different composition.
The layers C are preferably themselves IR-ablative. However, the
minimum requirement is that additional layers of this type must not
interfere with the ablation of A and B, i.e. must at least be
removable together therewith. All layers should furthermore be
soluble or at least swellable in the developer for the
photopolymerizable layer.
[0053] The sequence of layers A and B and any further layers is
essentially opaque to actinic radiation. In general, the optical
density of all IR-ablative layers together in the actinic spectral
region is at least 2.5, preferably at least 3.0, and very
particularly preferably 3.5. Said optical density is in each case
measured as the wavelength or in the wavelength range employed for
the exposure of the element during full-area irradiation. It is
entirely possible for the optical density of the individual layers
to be different and is generally less than 2.5 considered
individually in each case. The properties of the entire layer
system are crucial.
[0054] The total thickness of all IR-ablative layers should be as
low as possible in order that the layer system can be removed as
efficiently as possible by means of laser radiation. The total
thickness of the layer sequence is restricted at the lower end
inasmuch as the layer sequence must have an optical density of at
least 2.5. In general, a thickness of from 1 to 10 .mu.m is
appropriate, without the invention being restricted to this range.
The thickness of the individual layers from which the layer system
is built up is determined by the person skilled in the art
depending on the desired properties. In general, however, none of
the individual layers has a thickness less than 20% of the total
thickness of the layer system, although the invention is not
restricted thereto. In the case of two layers A and B, a thickness
ratio of from 60:40 to 40:60 has proven successful.
[0055] It is advisable to protect the IR-ablative flexographic
printing element according to the invention against damage during
transport, storage and handling by a flexible protective film,
although a protective film of this type is not absolutely
necessary.
[0056] The photosensitive flexographic printing elements according
to the invention can be produced by firstly applying the
photopolymerizable layer and optionally further layers to the
support, and subsequently applying the IR-ablative layers A and B
and, if desired, other layers, one after the other, for example by
casting or lamination. To this end, firstly all constituents of
each layer are dissolved or dispersed in a suitable solvent or
solvent mixture with vigorous mixing. Solutions or dispersions of
this type can then be applied directly to the photopolymerizable
element layer by layer and the solvent evaporated. However, the
solutions can also be cast onto a support film, for example a PET
film, layer by layer and the respective solvent evaporated. The
coated support is then laminated onto the photopolymerizable layer
under pressure and/or the influence of heat. The support for the
IR-ablative layers then functions as protective film for the entire
photopolymerizable printing element. Of course, with this
technique, the topmost layer must be cast first, followed by the
other layers.
[0057] The photosensitive printing elements according to the
invention with a plurality of IR-ablative layers A and B are
employed for the production of flexographic printing plates. If
present, the protective film is firstly removed. The IR-ablative
layer is then irradiated imagewise using a suitable laser in order
to obtain a photographic mask. Examples of suitable IR lasers
include Nd/YAG lasers (1064 nm) and diode lasers (for example 830
nm). Suitable laser systems for computer-to-plate technology are
commercially available, for example the OmniSetter.RTM. diode laser
system (Misomex, wavelength 830 nm; working width 1800 mm) or the
digilas.RTM. Nd/YAG laser system (Ohio-Schepers), each of which
comprises a cylindrical drum, onto which the flexographic printing
element with IR-ablative layer is mounted. The drum is subsequently
set in rotation, and the element is imaged by means of the laser
beam.
[0058] After writing the mask, the photosensitive element is
exposed over the entire area to actinic light through the mask.
This can advantageously be carried out directly on the laser
cylinder. However, the plate can also be taken off and exposed in a
conventional flat-bed exposure apparatus--in air, under nitrogen or
in vacuo. During the exposure step, the layer is photochemically
crosslinked in the areas uncovered by IR ablation in the preceding
process step, while the areas still covered remain
uncrosslinked.
[0059] In a further process step, the exposed element is developed.
The development can in principle be carried out in commercially
available developer apparatuses using commercially available
wash-out agents for flexographic printing plates, for example
nylosolv.RTM. or Optisol.RTM.. During the development, the
unexposed areas of the photopolymerizable layer and the residues of
the IR-ablative layer are removed. It is preferred to employ only a
single wash-out agent for this process step. However, it is also
possible firstly to remove the residues of the IR-ablative layer
using a wash-out agent and subsequently to develop the plate in a
second wash-out agent. After the development, the resultant
flexographic printing plates are dried in a known manner. The
process may comprise further process steps, for example
detackifying with UV-C or Br.sub.2.
[0060] The flexographic printing elements according to the
invention with a plurality of IR-ablative layers have a number of
advantages:
[0061] Although the sensitivity to laser radiation does not reach
that of pure nitrocellulose layers, it is nevertheless higher than
that of commercially available plates. The combination of an
elastomeric layer A with a layer comprising self-decomposing binder
B, such as, in particular, nitrocellulose, means that firstly a
very sensitive flexographic printing element is obtained which
nevertheless has surprising high elasticity. Flexographic printing
plates of this type can easily be laid around the drums of laser
exposure units without the layer tearing or wrinkling. The amount
of plasticizer in the nitrocellulose layer can be significantly
reduced compared with pure nitrocellulose plates. The advantages
are particularly clear if the IR-ablative layer comprising the
elastomeric binder is located at the uppermost point. The layer
surface is tack-free and the protective or cover film adhesion is
low, enabling the protective film to be removed without problems.
The adhesion to the photopolymerizable layer is high, so that the
IR-ablative layer remains adhering to the photopolymerizable layer
even on rapid and unintentional removal of the protective film. The
layers can be washed off using conventional organic wash-out
agents, such as nylosolv.RTM..
[0062] The following examples are intended to explain the invention
by way of example.
[0063] General procedure for the preparation of a dispersion of IR
absorber and binder.
[0064] A dispersion aid (Solperse 20 000) is pre-dissolved in the
solvent mixture. The binder(s) A and any further additives are
subsequently added and dissolved with stirring. The carbon black is
then added slowly and dispersed in for 15 minutes at a dissolver
speed of 5000 rpm. The resultant mixture is ground for 1 hour in a
laboratory stirred ball mill. The dispersion has a solids content
of 10% by weight, based on the sum of all constituents.
[0065] A dispersion of the solid constituents of layer B is
obtained in a similar manner in a suitable solvent.
[0066] Firstly, either dispersion A or dispersion B is coated onto
a PET film using a laboratory knife coater and dried at 60.degree.
C. for 10 minutes. The remaining dispersion is then introduced and
dried under the same conditions.
[0067] The optical density of a film of this type in the spectral
region of actinic light is from 3.0 to 3.5.
[0068] The protective film and substrate layer are removed from a
commercial flexographic printing plate of the FAH 254 type (BASF),
so that the surface of the photopolymerizable layer is uncovered.
The PET film coated with the IR-ablative layer system is
dry-laminated onto the surface of the photopolymerizable layer so
that the IR-ablative layer system and the photopolymerizable layer
are bonded to one another. The PET film forms the protective film.
The plate is stored for 1 week at room temperature.
EXAMPLES 1
[0069] A dispersion B for the IR-ablative layer B was prepared as
described from 35% by weight of carbon black, 3% by weight of
dispersion aid (Solperse 20000), 52% by weight of nitrocellulose
(type A 400, Wolff-Walsrode) and 10% by weight of plasticizer
(Plasthall 41 41 (CP Hall)) (or data based on the total amount of
all constituents of the dried layer) in a solvent mixture
comprising toluene, propanol, benzyl alcohol and ethyl acetate
(solids content 10% by weight).
[0070] A dispersion A for the IR-ablative layer A was prepared as
described from 35% by weight of carbon black, 3% by weight of
dispersion aid (Solperse 20000), 62% by weight of an elastomeric
binder (elastomeric polyether-polyurethane), Surkofilm 72 S,
Mitchanol) (or data based on the total amount of all constituents
of the dried layer) in a solvent mixture comprising ethanol, ethyl
acetate and benzyl alcohol (solids content 10% by weight). The
starting materials and amounts are also shown in Table 1.
[0071] Firstly dispersion A was cast onto the PET film and dried,
then dispersion B. The layer system obtained was laminated onto the
flexographic printing plate as described. The thickness of the
layer system was about 2.5 .mu.m; the two part-layers were of equal
thickness.
[0072] The cover film adhesion, the substrate adhesion, the
elasticity and the tack of the IR-ablative layer of the resultant
plates were determined. The measurement methods for the measurement
parameters are shown in Table 3, and the results are shown in Table
2.
EXAMPLES 2 AND 3
[0073] The procedure was as in Example 1, but different elastomeric
binders were employed.
COMPARATIVE EXAMPLE C1
[0074] The procedure was as in Example 1, but layer A was omitted,
and only the nitrocellulose-containing layer B was cast. The
thickness of layer B was about 2.5 .mu.m.
COMPARATIVE EXAMPLE C2
[0075] The procedure was as in Example C1, but the alcohol-soluble
nitrocellulose type A 400 was replaced by the ester-soluble type E
900 (each Wolff-Walsrode).
[0076] Laser Imaging, Conversion into a Flexographic Printing
Plate, General Procedure
[0077] The PET protective film is removed from the flexographic
printing element with IR-ablative layer system, and the plate is
mounted onto the rotatable drum (diameter: 0.25 m) of an Nd/YAG
laser system (Schepers, Ohio, digilas.RTM.). The drum is
accelerated to 1600 rpm. The laser moves forward by 10 .mu.m per
drum rotation.
[0078] In a first experiment, firstly the minimum laser power
necessary for complete removal of the IR-ablative layer at the
points at which it is hit by the laser beam is determined for said
drum speed and laser advance for each plate. The requisite laser
power can be used to determine the sensitivity of the plate in
J/m.sup.2, i.e. the minimum energy per unit area which is necessary
to remove the layer. The sensitivity is higher the lower the
minimum energy.
[0079] In a further experiment, each plate is imaged digitally with
the laser power determined. The photopolymerizable flexographic
printing element obtained with digital mask is irradiated for 15
minutes with actinic light in a conventional manner and
subsequently developed in a conventional manner in a brush washer
using the commercial (organic) wash-out agent nylosolv.RTM. II.
After the development, the plate is dried at 60.degree. C. for 2
hours, irradiated with UV-C light for detackifying and finally
post-crosslinked for 10 minutes with actinic light.
EXAMPLES 1-3 AND COMPARATIVE EXAMPLES C1-C2
[0080] In each case, the flexographic printing elements produced
were employed. The laser power determined in each case and the
sensitivity are shown in Table 2. The ability of the IR-ablative
layer to be washed off or removed in the wash-out agent was also
determined. The results are shown in Table 2, and the measurement
method is shown in Table 3.
COMPARATIVE EXAMPLES C3-C6
[0081] The mechanical properties, the sensitivity and the
processing properties of 4 commercially available flexographic
printing elements with IR-ablative layer were determined as
outlined above. The results are likewise shown in Table 2.
1TABLE 1 Compositions of the IR-ablative layers No. Layer A Layer B
Sequence 1 62% of polyether-polyurethane (Surkofilm 72 S) 52% of
nitrocellulose (A 400), 10% of plasticizer, 35% of B-A 35% of
carbon black, 3% of Solperse carbon black, 3% of Solperse 2 62% of
SBS polymer (Kraton .RTM. 1101) 52% of nitrocellulose (A 400), 10%
of plasticizer, 35% of B-A 35% of carbon black, 3% of Solperse
carbon black, 3% of Solperse 3 62% of elastomeric polyamide
(Makromelt .RTM. 6900) 52% of nitrocellulose (A 400), 10% of
plasticizer, 35% of B-A 35% of carbon black, 3% of Solperse carbon
black, 3% of Solperse C1 No layer A 52% of nitrocellulose (A 400),
10% of plasticizer, 35% of Only one layer carbon black, 3% of
Solperse C2 No layer A 52% of nitrocellulose (A E 950), 10% of
plasticizer, 35% of Only one layer carbon black, 3% of Solperse
[0082]
2TABLE 2 Test results Laser Sensi- Ability to be power tivity Tack-
Substrate Cover film washed off in No. Product [W] [J/cm.sup.2]
free adhesion adhesion Elasticity nylosolv II 1 Layer A: Polyether
polyurethane Surkofilm 72 S 3.0 1.5 + + + + + Layer B:
Nitrocellulose/plastici- zer 2 Layer A: SBS rubber 3.2 1.6 + + + +
+ Layer B: Nitrocellulose/plasticizer 3 Layer A: Polyamide 3.2 1.6
+ + + (+) + Layer B: Nitrocellulose/plasticizer V 1 Only one layer
made of NC A 400/ 1.7 0.8 + + - (-) + plasticizer V 2 Only one
layer made of NC E 950/ 1.8 0.8 + + (-) (+) (+) plasticizer
Commercial product V 3 Dupont Cyrel .RTM. DPH 5.0 2.4 + (+) + (+) +
V 4 Asahi AFP .RTM. XDI 4.3 2.1 + + + (+) + V 5 BASF digiflex .RTM.
I 3.7 1.8 + (+) + (+) - V 6 Polyfibron Flexlight .RTM. CBU 6.3 3.1
+ + + (+) + Layers each contain 35% of carbon black
[0083]
3TABLE 3 Explanations of the assessments in Table 1
Property/Assessment + (+) (-) - Tack-free Fully tack-free Low tack
Slightly tacky Highly tacky Substrate adhesion Very strong:
Substrate difficult to Substrate easy to remove: Weak, substrate
very easy to substrate cannot be remove: 0.3-2. remove 0.1-0.2
removed: >30 and very 2-5 difficult to remove: 5-30 Cover film
adhesion Weak Normal Strong Too strong/very difficult to 0.01-0.08
0.09-0.19 0.2-0.5 remove: >0.5 Elasticity Does not tear and does
not Does not tear and does not Tears and wrinkles on Tears easily
and wrinkles on wrinkle even on severe wrinkle on external external
and internal external and internal bending inward and bending, but
does on bending bending outward internal bending Ability to be
washed out in Removed after 1st brush Removed after 2nd brush
Removed after 3rd brush Remains on the nylosolv .RTM. II
flexographic plate during the entire washing process Values for
substrate adhesion and cover film adhesion in N/4 cm: N/4 cm:
Measured on a 4 cm wide strip of the printing plate in a Zwick 1435
universal testing machine for stress/strain experiments. Ability to
be washed out: during standard wash-out operation in a nyloflex
Combi L F II wash-out unit
[0084] The examples and comparative examples show that flexographic
printing elements are obtained which satisfy the requirements in an
excellent manner. The flexographic printing elements according to
the invention comprising at least two IR-ablative layers
furthermore have the advantage that alcohol-soluble nitrocellulose
grades, which are also very readily soluble in customary developers
for flexographic printing plates, can also be employed here. Owing
to their high top film adhesion, the use of grades of this type was
not possible in the case of the known single-layered systems. The
invention opens up the possibility of also employing grades of this
type.
* * * * *